What can I legally do on a severe case of assault on school grounds?

My daughter was sexually and physically assaulted on school grounds repeatedly over the period of 2 years. This happened over 4 years ago but I only recently learned the extreme nature of the bullying when she developed PTSD and a disassociative disorder as a result of the trauma and can no longer attend school. She had no prior mental health issues and was an honor roll student. I know the staff knew something was going on and that other kids saw, but I was never notified.